Mexico City -LRB- CNN -RRB- -- Workers digging through the rubble of what were offices at Mexico 's state-run oil giant have found more victims , raising the death toll from last week 's blast to 35 .

Pemex , the oil monopoly , on Sunday confirmed the figure , which was above the 33 reported killed earlier .

More than 100 people were injured ; two people remain missing .

Pemex , the powerful but much-criticized state oil company , has said it 's not clear what caused the blast , which took place Thursday at an administrative building in the Pemex office complex in Mexico City .

Authorities have said investigations are under way to determine whether the explosion was the result of an accident , negligence , or an attack .

`` People were screaming . ... You could see pieces of the wall falling to the ground , '' said Joaquin Borrell Valenzuela , an attorney for the Pemex comptroller 's office , who was in a courtyard outside the building at the time of the blast .

Paramedics quickly arrived and started pulling people from the debris .

Thousands of people work at the Pemex headquarters , which includes a 54-story building that is nearly 700 feet tall . The annex where the explosion happened is adjacent to the tower .

Mexican oil giant chief wo n't speculate on cause of deadly blast

CNN 's Mauricio Torres contributed to this report .

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At least 35 people were killed in last week 's explosion

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Two people remain missing

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The blast happened at the Mexico City offices of Pemex , the state-run oil giant

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Investigations are under way